Quantitative aspects of electrolysis in electromembrane extractions of acidic and basic analytes

Electrolysis is omnipresent in all electrochemical processes including electromembrane extraction (EME). The effects of electrolysis on quantitative aspects of EME were comprehensively evaluated for a set of acidic (substituted phenols) and basic (basic drugs) analytes. EMEs were carried out across supported liquid membranes formed by 1-ethyl-2-nitrobenzene at standard EME conditions, i.e., acidic analytes were extracted from alkaline into alkaline solutions and basic analytes were extracted from acidic into acidic solutions. Electric potential applied across the EME systems was 50 V and extraction recoveries of analytes as well as pH values of donor and acceptor solutions were determined after each EME. It has been proven that electrolysis plays a more significant role than has ever been thought before in EME. Electrolytically produced H⁺ and OH^– ions had a significant effect on pH values of acceptor solutions and variations of up to 8.5 pH units were obtained at standard EME conditions. pH values of donor solutions were affected only negligibly due to their significantly higher volumes. The observed variations in pH values of acceptor solutions had fatal consequences on quantitative EME results of weak and medium strong acidic/basic analytes. A direct relation was observed between the decrease in extraction recoveries of the analytes, their pK_a values and the acceptor solution pH values. Acceptor solutions consisting of high concentrations of weak bases or acids were thus proposed as suitable EME operational solutions since they efficiently eliminated the electrolytically induced pH variations, offered stable EME performances and were easily compatible with subsequent analytical methods.     

Organic acid solutions in the chromatography of inorganic ions

Summary We present a study of the chromatographic behaviour of sixty ions on thin layers of cellulose, employing as eluents aqueous solutions of tartrate, at various molarities (from 0.1 to 1.0 mol dm^–3 and pH values (from 2.0 to 10.0 obtained with aqueous ammonia). The ions migrate, whether they are complexed with the tartrate or not, with high R_F values and are not influenced by variation of the tartrate concentration. The pH variations, owing to the formation of partial hydrolysis or precipitation products or possibly ammonia complexes, have a greater effect on the rate of ion migration and cause a decrease in the R_F values. We show interesting separations obtained with these eluents.This work has been in part supported by C.N.R. of Italy     

Absorption and fluorescence behavior of cytosine irradiated at different pH and in the presence of Cu(II) ions at neutral pH

Dilute aqueous solutions of cytosine were irradiated with⁶⁰Co -rays under N₂O saturated conditions at different pH and in the presence of Cu(II) ions at neutral pH. The base degradation decreased from neutral to acidic and basic conditions. In the presence of metal ions at neutral pH conditions there was a significant increase in the base degradation compared to that in the absence of metal ions under similar conditions. From the difference absorption spectra and fluorescence behavior of the irradiated solutions it was observed that the major radiolytic products of cytosine under different conditions are cytosine glycols, 5-hydroxycytosine, hydroxy-hydrocytosine and cytosine dimers. The yields of dimers is maximum in neutral conditions and it decreased from basic to acidic conditions. However, in the presence of Cu(II) ions formation of cytosine dimers is completely restricted and there is an increase in the yields of cytosine glycol, hydroxy-hydrocytosine and 5-hydroxycytosine. From the post-radiolytic changes in absorption and fluorescence behavior of irradiated solutions, it is revealed that some of the radiolytic products, namely cytosine glycol and hydroxy-hydrocytosine decompose to 5-hydroxycytosine and cytosine, respectively.     

Study of the aggregation of human insulin Langmuir monolayer

The human insulin (HI) Langmuir monolayer at the air-water interface was systematically investigated in the presence and absence of Zn(II) ions in the subphase. HI samples were dissolved in acidic (pH 2) and basic (pH 9) aqueous solutions and then spread at the air-water interface. Spectroscopic data of aqueous solutions of HI show a difference in HI conformation at different pH values. Moreover, the dynamics of the insulin protein showed a dependence on the concentration of Zn(II) ions. In the absence of Zn(II) ions in the subphase, the acidic and basic solutions showed similar behavior at the air-water interface. In the presence of Zn(II) ions in the subphase, the surface pressure-area and surface potential-area isotherms suggest that HI may aggregate at the air-water interface. It was observed that increasing the concentration of Zn(II) ions in the acidic (pH 2) aqueous solution of HI led to an increase of the area at a specific surface pressure. It was also seen that the conformation of HI in the basic (pH 9) medium had a reverse effect (decrease in the surface area) with the increase of the concentration of Zn(II) ions in solution. From the compression-decompression cycles we can conclude that the aggregated HI film at air-water interface is not stable and tends to restore a monolayer of monomers. These results were confirmed from UV-vis and fluorescence spectroscopy analysis. Infrared reflection-absorption and circular dichroism spectroscopy techniques were used to determine the secondary structure and orientation changes of HI by zinc ions. Generally, the aggregation process leads to a conformation change from α-helix to β-strand and β-turn, and at the air-water interface, the aggregation process was likewise seen to induce specific orientations for HI in the acidic and basic media. A proposed surface orientation model is presented here as an explanation to the experimental data, shedding light for further research on the behavior of insulin as a Langmuir monolayer.     

Über die Chemie der Oberfläche des Titandioxids. IV. Austausch von Hydroxidionen gegen Fluoridionen

On reaction of anatase of high surface area with NaF solutions, OH groups of the TiO₂ surface were replaced by F^?. Only half the surface OH groups were exchanged in unbuffered solutions or at a pH at or slightly above the zero point of charge (pH 6.6). These are the OH groups exhibiting basic character. The remaining OH groups of acidic character exchange H⁺ for Na⁺ concurrently. They too, are replaced by fluorine in more acidic solutions (pH 4.6).     

The passivity of nickel in phosphate solutions: Part II. Effect of solufion pH

The passivity of nickel has been studied over a wide range of pH (1.5–12.6) in phosphate solutions. The results show that solution pH has a strong effect on the formation and the growth of the passive films. In basic solutions (pH 9.1 and 12.6) and in moderately acidic solution (pH 4.5), the passive film possess the came optical constants but exhibit different growth behaviors. The film formed in the basic solutions does not grow, but the film formed in the pH 4.5 solution increases in thickness with time. In an acidic solution of 1.5 pH, the passive film is composed of two layers: a base layer of partially dehydrated Ni(OH)₂, and an upper, potential-dependent film. The two-layer film evolves with time.     

Insight into the degradation of a manganese(III)-citrate complex in aqueous solutions

Kinetics of the electron transfer process between citrates and manganese(III) ions has been studied in acidic aqueous solutions. Acidification of the reaction mixture increased the reaction rate. The reaction is dependent on pH because there are two main protolytic forms of the Mn(III)-citrate complex in the studied pH range (4.5–6.5). Reduction potentials of Mn(III)/Mn(II) system in acidic and basic solutions as well as protolytic equilibria play a crucial role in understanding the pH profile of the studied system. The rate constants for Mn(III)citH and Mn(III)citH₂⁺ species degradation processes are presented (citH³⁻ and citH₂²⁻ are trivalent and divalent anions of citric acid, citH₄, respectively). Protolytic constant (expressed as pK′_a) for Mn(III)citH protonation is estimated and discussed.     

ESI-MS study of the mechanism of glycyl-l-histidyl-l-lysine-Cu(II) complex transport through model membrane of stratum corneum

In mammalian organisms copper can be found mainly in the form of complex with specific tripeptide, GHK-Cu (glycyl-l-histidyl-l-lysine-Cu(II)). GHK-Cu is the basic form in which copper is transported in tissues and permeates through cell membranes. The penetration ability of GHK-Cu through the stratum corneum and its role in copper ions transport process is the key issue for its cosmetic and pharmaceutical activity. The permeability phenomenon was studied by use in vitro model system—Flynn diffusion cell with the liposome membrane.The earlier studies on the influence of different ligands on the migration rate of copper ions through model membrane provide evidence for hampering role of ligands structure and pH of formulations in this process.Structures of copper complexes formed in solutions of different pH media were evaluated by use of ESI-MS. The permeability coefficients of copper complexes increase with increasing pH. It was proved that only tripeptide GHK and its complexes with copper: GHK-Cu and (GHK)₂-Cu are able to migrate through membrane model of stratum corneum.     

Ligand-exchange chromatography of amino acids on nickel-Chelex 100.

A ligand-exchange chromatographic proceudre for the selective separation of amino acids from inorganic ions is presented. It was found that the binding of amino acids to the nickel-Chelex 100 resin is pH dependent. At pH 8.5-9.1, only the basic amino acids lysine, histidine and arginine are quantitatively attached to the complex, whereas at pH 11, other amino acids with the exception of aspartic acid and glutamic acid are also bound, although not quantitatively. All of the amino acids can be eluted from the complex with 3 M ammonia solution without the displacement of nickel ions from the complex. This method can be used for the removal of the basic amino acids from solutions in the presence of inorganic ions as well as other amino acids.     

Kinetics and mechanism of amitraz hydrolysis in aqueous media by HPLC and GC-MS

Degradation processes of amitraz in aqueous media have been studied by spectrophotometry, HPLC and GC-MS. Amitraz undergoes hydrolysis reactions at any pH, but towards the acidic pH range hydrolysis proceeds at a faster rate. Depending on the pH value, different products of the hydrolysis have been identified. The main degradation products are 2,4-dimethylaniline at very acidic pH values (pH<3), N-(2,4-dimethylphenyl)-N'-methylformamidine and 2,4-dimethylphenylformamide at less acidic media (pH 3-6) and 2,4-dimethylphenylformamide at basic pH. The mechanisms of the different hydrolysis processes have been elucidated.     

Synthetic Hydroxyapatites as Inorganic Cation-Exchangers; Exchange Characteristics for Pb2+ and Sn2+ Ions in Acidic Solution

Abstract

The cation-exchange characteristics between Pb²⁺ ions of aqueous solutions containing counter-anions (F^?, C1^?) and Ca²⁺ ions of synthetic hydroxyapatite samples have been investigated in detail under the conditions of low pH values (3.0, 4.0 and 5.0) by a normal batch method. Even at the low pH value of 3.0 the apatite structure in a solution containing F^? or C1^? ions was maintained via a concurrent ion-exchange effect of Pb²⁺ ions together with F^? or C1^?ions, which are known to be exchangeable with OH- ions of the apatite. Moreover, it was found that Ca²⁺ ions in the apatite sample can easily be exchanged for Pb²⁺ ions almost without distinction between MI and M2 sites, assisted by the loosening effect of protons even at room temperature. Next, it was found that the hydroxyapatite samples are transformed into amorphous states by the reactions between Ca ²⁺ ions in the samples and Sn ²⁺ ions in the SnC1₂ acidic aqueous solutions with pH of 3.0 or below with a molar ratio of Sn²⁺/Ca²⁺ -1.0. The existence of hydroxyapatite as amorphism in acidic aqueous solutions such as SnC1₂ is quite interesting, because in general the hydroxyapatite has been found to be dissolved in acidic aqueous solutions. Moreover, the obtained amorphism are found to be stable up to at least 500 to 6OO°C but to be unstable in alkaline solutions. The characteristics of Sn²⁺ ions are found to have been found to form crystalline Pb ²⁺ apatite even in be quite different from those of homologous Pb²⁺ ions which have been found to form crystalline pb²⁺ apatite even in such an acidic atmosphere.     

Electrocatalysis and pH (a review)

The factors determining pH effects on principal catalytic reactions in low-temperature fuel cells (oxygen reduction, hydrogen oxidation, and primary alcohols oxidation) are analyzed. The decreasing of hydrogen oxidation rate when passing from acidic electrolytes to basic ones was shown to be due to the electrode surface blocking by oxygen-containing species and changes in the adsorbed hydrogen energy state. In the case of oxygen reduction, the key factors determining the process’ kinetics and mechanism are: the O₂ adsorption energy, the adsorbed molecule protonation, and the oxygen reaction thermodynamics. The process’ high selectivity in acidic electrolytes at platinum electrodes is caused by rather high Pt-O₂ bond energy and its protonation. The passing from acidic electrolytes to basic ones involves a decrease in the oxygen adsorption energy, both at platinum and nonplatinum catalysts, hence, in the selectivity of the oxygen-to-water reduction reaction. The increase in the methanol and ethanol oxidation rate in basic media, as compared with acidic ones, is due to changes in the reacting species’ structure (because of the alcohol molecules dissociation) on the one hand, and active OH_ads species inflow to the reaction zone, on the other hand. In the case of ethanol, the above-listed factors determine the process’ increased selectivity with respect to CO₂ at higher pHs. Based on the survey and valuation, priority guidelines in the electrocatalysis of commercially important reactions are formulated, in particular, concepts of electrocatalysis at nonplatinum electrode materials that are stable in basic electrolytes, and approaches to the practical control of the rate and selectivity of oxygen reduction and primary alcohols oxidation over wide pH range.     

Interfacial Tension Measurements Between Oil Fractions of a Crude Oil and Aqueous Solutions with Different Ionic Composition and pH

Dynamic and equilibrium interfacial tensions between crude oil fractions and aqueous solutions of various compositions and pH were measured. The basic oil components seemed to determine the interfacial tensions at pH 2, while the non-dissociated and dissociated acidic components governed the interfacial tension at the natural pH and pH 9, respectively. The ionic composition of the aqueous phase influenced the degree of dissociation of the acidic components at pH 9: Na⁺ ions in the aqueous phase promoted dissociation of the interfacial acidic components (compared to pure water), while Ca²⁺ ions resulted in complexation with the dissociated acids and most likely formation of stable interfacial films. The amount of Ca²⁺ determined which of these phenomena that dominated when both ions were present in sea water solutions. Generally, the interfacial tensions of the oil fractions were lower when measured against the high salinity aqueous solutions than against the corresponding low salinity solutions.      

Phenol Photonitration and Photonitrosation upon Nitrite Photolysis in basic solution

Nitrophenols have been detected in some Antarctic lakes, the water of which is basic and rich in nitrate, nitrite and other nutrients. Nitrate or nitrite photolysis could be a possible reaction to explain the presence of these compounds. This work presents evidence for the formation of 2-nitrophenol (2NP), 4-nitrophenol (4NP) and 4-nitrosophenol (4NOP) upon UV irradiation of phenol and nitrite in aerated basic solutions.

The pH dependence of the 2NP initial formation rate is different from those of 4NP and 4NOP. The dependence of the first mainly reflects the phenol/phenolate equilibrium, with phenol yielding 2NP at a higher rate than phenolate. In the case of 4NOP, the initial formation rate vs pH has a maximum at pH 9.5. The pH dependence of 4NOP formation rate suggests that three pathways are likely to operate: nitrosation of undissociated phenol by N₂O₃, prevailing at pH<8.7, nitrosation of phenolate by N₂O₃, prevailing in the pH interval 8.7–10.8, and reaction between phenoxyl radical and ^?NO, prevailing at pH>10.8. Phenol nitrosation by N₂O₃ is favoured when phenol is negatively charged (phenolate), but it is also disfavoured at alkaline pH values, owing to the depletion of N₂O₃ (the nitrosating agent) by basic hydrolysis. Differently from 2NP, the initial formation rate vs pH of 4NP is very similar to that of 4NOP, suggesting that 4NP may originate from the oxidation of 4NOP. Moreover, while in neutral and acidic solutions the formation rate of 2NP is slightly higher than that of 4NP, in the pH interval 8–12 the formation of 4NP is much more rapid than that of 2NP. This indicates that the pH of natural waters influences the ratio of nitroisomers.     

Photoexcitation of the potassium octacyanotungstate(IV)· 5–nitro-1,10–phenanthroline system: kinetics and mechanism of the reaction

The kinetics of thermal reactions of the photochemically-generated species [W(CN)7OH]4− and [W(CN)6–(CNH)(H2O)]2− with 5–nitro-1,10–phenanthroline (nitrophen) in basic and acidic media, respectively, were studied in buffer solutions at pH 4.2–10.6 and ionic strength 7.5×10−2kg−1 at 20°C. The quantum yield for the formation of the photoproduct was calculated and found to depend upon pH and the ligand and [W(CN)8]4− concentrations. The rate constants and quantum yields were less compared with 1,10–phenanthroline due to the electron-withdrawing inductive effect of the nitro group in nitrophen, which makes the latter a weaker ligand. The pseudo-first-order rate constant and quantum yield values in acidic media are higher than in basic media and the mechanisms of the photochemical substitution are different in the two media. This revised version was published online in June 2006 with corrections to the Cover Date.     

Inverse emulsion polymerization of acrylamide. II. Synthesis and characterization of copolymers with methacrylic acid

Inverse emulsion copolymerization studies of acrylamide (Am) with methacrylic acid (MAA) are reported. Aqueous monomer solutions were emulsified in toluene with a blend of two surfactants (sorbitan sesquioleate and C18-terminated acrylamide oligomers). Polymerization kinetics in presence of an oil-soluble initiator (AIBN) were determined at 40°C as a function of methacrylic acid content and aqueous-phase pH. Polymerization rates were found to be faster at basic pH than at acidic pH, which appeared to be related to the actual concentration of methacrylic acid in the aqueous phase. Monomer reactivity ratios have been derived as r_A-M = 0.58 ± 0.02 and r_M-A = 4.0 ± 0.10 at pH 4, r_A-M = 0.56 ± 0.005 and r_M-A = 0.15 ± 0.03 at pH 10. These differences were found to have an effect on the molecular characteristics of the copolymers. Initial emulsions and final inverse latexes displayed the same broad size distribution; under basic pH the particle size is relatively insensitive to the ionic comonomer concentration. Poor latex stability is characteristic of copolymer latexes prepared under acidic conditions. Based on these experimental results, some aspects of the polymerization mechanism are discussed.     

Study by electron paramagnetic resonance and spectrophotometry of absorption of the primary processes in photochemistry of frozen aqueous solutions at 77 degree K of aromatic amino acids and polypeptides

Abstract— Neutral, acidic or basic frozen aqueous solutions of aromatic amino acids undergo photoionisation under u.v. irradiation, at 77°K. In neutral or basic solutions, photo-ejected electrons are trapped in the solvent matrix and exhibit a characteristic absorption band in the visible region. In acidic solutions electrons are trapped by protons and ESR signal spectrum of hydrogen atoms may be observed. Hydrogen atoms are also produced in low yield in neutral or basic frozen aqueous solutions, u.v. irradiated at 77°K. In basic media the ESR spectrum of 0^- radical ions is observed. Kinetic studies as a function of light intensity show that photoionisation takes place after absorption of a second photon by the phosphorescent molecule in its lowest triplet state. Recombination of trapped electrons in neutral or basic solutions may be induced using secondary excitation with visible light. In all instances we could record the absorption spectrum of photolytic products of aromatic amino acids and polypeptides which are u.v. irradiated at 77°K.     

Redox Reactions of Quercetin and Quercetin-5'-sulfonic Acid with Fe3 + and Cu2 + Ions and with H2O2

Redox reactions of quercetin and quercetin-5'-sulfonic acid with Fe^{3 +} and Cu^{2 +} ions and with H₂O₂ were studied spectrophotometrically. Oxidation of the flavonoids occurs at the 3-OH and 4-OH groups. The redox reactions are largely influenced by pH. With Fe^{3 +} ions, oxidation occurs in strongly acidic (pH 1-2), and with Cu^{2 +} ions, in weakly acidic (pH 4-5) solutions. Oxidation of quercetin and quercetin-5'-sulfonic acid with Fe^{3 +} and Cu^{2 +} ions is accompanied by complexation. Hydrogen peroxide oxidizes the flavonoids at pH 1-3.5, and at pH > 4 oxidation is insignificant.     

Investigation of fulvic acid photochemistry in aqueous solutions by stationary and laser flash photolysis techniques

Photochemistry of fulvic acid (FA, Henan ChangSneng Corporation) in aqueous solutions was studied using stationary and nanosecond laser (355 nm) flash photolysis. UV-excitation leads to formation of FA triplet state which is characterized by wide unstructured absorption band with maximum at 620 nm. The yield of FA triplet state depends on pH: intermediate absorption signal is maximal at neutral pH (6–7) and decreases in basic and acidic media. Kinetics of triplet state decay does not depend on solution pH and exhibits multiexponential character with characteristic times t ₁ = 4.3 ± 2.2, t ₂ = 54 ± 28, t ₃ = 830 ± 240 μs.     

Protonation in electrospray mass spectrometry: Wrong-way-round or right-way-round?

The term “wrong-way-round ionization” has been used in studies of electrospray ionization to describe the observation of protonated or deprotonated ions when sampling strongly basic or acidic solutions (respectively) where such ions are not expected to exist in appreciable concentrations in solution. Study of the dependence of ionization of the weak base caffeine on the electrospray capillary potential reveals three distinct contributors to wrong-way-round ionization. At near-neutral pH in solutions of low ionic strength, protonation of caffeine results from the surface enrichment of electrolytically produced protons in the surface layer of the droplets from which ions are desorbed. For solutions made strongly basic with ammonia, gas-phase proton transfer from ammonium ions can create protonated caffeine. These two mechanisms have been discussed previously elsewhere. For solutions of high ionic strength at neutral or high pH, the data suggest that discharge-induced ionization is responsible for the production of protonated caffeine. This mechanism probably accounts for some of the wrong-way-round ionization reported elsewhere.